Bull. Inst. Natn. Scien. Tech. Mer de Salammbô, Vol. 38, 2011
POSIDONIA OCEANICA MEADOWS ALONG THE EASTERN COAST OF TUNISIA: FEATURE AND HEALTH STATUS
Yassine Ramzi SGHAIER*, R. ZAKHAMA-SRAIEB and F. CHARFI-CHEIKHROUHA Unité de Recherche de Bio-écologue et Systématique Evolutive, Faculté des Sciences de Tunis, 2092, Manar II, Tunisie * [email protected]
ا ات ا ت ا ز و ا ا د ا ˸ درا ا آ وا و ت ا ز و أر ا ا ا د ا وه ه وا وي وا وا . آ م ا ز و و ا وا ا ت و ا و ه ا إ إ ف ا ا ي ه ا ا . ا وا ا ت ا ز و ا او ا 441 و 984 ز و ا ا ا ا اوح ا ا ر 8.5 إ 13 ا ز و ا ا ا ا إ د ا وراق ا ا ا وا ق . و ع ا ز و ا وه ر وا وي إذا ا ر ا ت ا م2 . ا وع آ ازداد ا آ ا وي وا . إن ه ا را م ت ة ا ا ا ت ا ز و ا ا ت . . آ ت ا : ا ب ا ، ت ا ز و ، آ م،
RESUME
Les herbiers à Posidonia oceanica au niveau du centre Est de la Tunisie : Caractéristiques et état de vitalité. La structure, la phénologie et la croissance des faisceaux des herbiers à Posidonia oceanica ont été étudiées dans quatre sites au niveau du centre Est de la Tunisie (Hergla, El Kantaoui, Monastir et Mahdia). La densité des faisceaux, le recouvrement et les paramètres phénologiques présentent une différence significative entre les sites et les profondeurs en relation avec l’intensité des activités humaines. Les herbiers les plus denses (441 à 984 faisceaux m-²) et les indices foliaires les plus élevés (8,5 à 13m2 m-²)) ont été enregistrés à Mahdia. Le nombre moyen des feuilles produites annuellement ne montre pas de différence significative entre les sites et les profondeurs étudiées. La croissance du rhizome de Posidonia oceanica est considérablement plus importante à Mahdia et à Hergla qu’à Monastir et à El Kantaoui à l’exception des stations les moins profondes (-2m), alors que la croissance des rhizomes et la profondeur sont négativement corrélées à El Kantaoui et à Monastir. L’étude fournit de nouvelles informations sur l'état de santé actuelle des herbiers à Posidonia oceanica au niveau d’une aire géographique peu étudiée. Mots clés: Phanérogames marines, Posidonia oceanica , densité des faisceaux, Lépidochronologie, Tunisie.
ABSTRACT
The structure, phenology and shoot growth of Posidonia oceanica meadows were assessed in four sites along the eastern Tunisian coast (Hergla, El Kantaoui, Monastir, Mahdia). The shoots densities, percentage of meadow cover and phenological studied parameters differed significantly between localities and depths according to human activities disturbances. P. oceanica developed the densest meadows (from 441 to 984 shoots m-²) and the highest value of Leaf Area Index (8.5 to 13 m 2 m-²) in Mahdia. There was no significant difference on the annual leaf production between the studied localities and sampling stations. Except at -2m depth, the rhizome growth was significantly greater at Mahdia and Hergla than Monastir and El Kantaoui. The rhizome growth was negatively related to the depth at El Kantaoui and Monastir. The study provides useful data on the state of health of P. oceanica in an area of the southern side of the Mediterranean Sea still for long time unknown. Keywords: Seagrasses, Posidonia oceanica , Shoot density, Lepidochronology, Tunisia.
INTRODUCTION important primary producers of the coastal zone in the Mediterranean basin (Ott, 1980). It forms Extensive beds of seagrasses provide valuable widespread meadows extending from the surface to resources in shallow coastal waters worldwide (Short about 40 m depth (Pérès and Picard, 1964) and covers and Neckles, 1999). The endemic seagrass Posidonia about 2.5±5 millions of hectares of the Mediterranean oceanica (L.) Delile represents one of the most (Pergent et al., 1995). P. oceanica meadows play a
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Bull. Inst. Natn. Scien. Tech. Mer de Salammbô, Vol. 38, 2011 key role in the Mediterranean Sea, hosting very domestic effluents produced by a population of diverse animal and algal communities (Mazzella et 100000 equivalent inhabitants (reaching 140000 al., 1989), and representing an important nursery during summer due to the increase of the touristic grounds for several species (Francour et al., 1999; activities due to the presence of tourists). The Sanchez-Jerez et al., 2000). Its role is important in the seawage is located at 0.8 MN from the coast and at 14 stability of coastal sediments and consequently in the m depth. The estimated outflow is about 0.3 m3 s-1, protection of beaches from erosion (Blanc and Jeudy fluctuated varies according to the season; the de Grissac, 1984; Jeudy de Grissac and suspended particulate matter concentration is 40 mg Boudouresque, 1985). Furthermore, P. oceanica is l-1 and a high concentration of organic carbon (more very sensitive to disturbances (Francour et al., 1999; than 291mg g-1) and micropollution was detected in Ruiz and Romero, 2003). Moreover, it is considered the sediment near the water treatment plant (Pavoni et as an excellent bioindicator (Boudouresque et al., al., 2000). 2000; Pergent-Martini and Pergent, 2000). Monastir is a coastal touristic city with a marina P. oceanica is very common along the Tunisian coast built in 1980 accommodating 440 boats in a 4 ha (Le Danois, 1925). However, as other cities in the marina and a fishing port (8 ha) built in 1984, 1.45 Mediterranean Sea, the marine costal ecosystem in MN south of the marina. Tunisia is deeply affected by the loss of seagrass meadows over extensive areas during the last decades, particularly, P. oceanica meadows (Ben Mustapha et al., 1999) and has been attributed to the impact from human activities. In fact, the most damaging impacts are industrial and domestic effluents and shoreline constructions enhancing an increase of nutrients and organic matter load and an imbalance in the sediment budget (Pergent and Kempf, 1993; Ben Mustapha et al., 1999) with direct N Hergla
aggressions as results from illegal trawling on those 35° El meadows (Ben Mustapha et al., 1999) 50 Kantaoui
The status of P. oceanica meadows along the eastern Monastir
Tunisia
Tunisian coasts is still poorly known; the aim of the Mediterranean Sea present study is to evaluate the health status of four
Tunisian P. oceanica meadows using structural, 35°30 morphological and shoot growth parameters in order Mahdia to detect differences among the meadows at different depths and potentially subjected to anthropogenic impact. 11 °30 11 °10 Figure 1. Sampling localities of Posidonia oceanica MATERIAL AND METHODS meadows.
Study site and sampling procedures Study was conducted by scuba diving in October Mahdia is a coastal tourist city with an important 2004 in four meadows along the eastern coast of fishing port (9.75 ha) established in 1967 and some Tunisia: Hergla, El Kantaoui, Monastir and Madhia. protected archaeological vestiges. Three stations were selected at each meadow in the Shoot density was estimated in situ at each station by following depths: -2, -5 and -10m (Fig. 1, Table I). counting the number of shoots present in a 40 cm x 40 cm quadrat with ten replicates. Percentage cover Hergla is a coastal village, with a small coastal port 2 (3.2 ha) constructed in 1984 and an offshore tuna was estimated using 1 m quadrat (divided into four farm installed in 2003 (Hattour, 2005). The farm 0.5m x 0.5m squares) with three replicates. Twenty P. produces 400-700 tons of fish per year and is situated oceanica orthotropic rhizomes were sampled at at approximately 2.7 MN from the shore over a random in each station for laboratory analysis. bottom depth of 40 m. Morphological and biometrical data El Kantaoui is among the most attractive touristic Leaf length and width were measured According to resort in Tunisia with many touristic plants (more Giraud (1979), leaves were detached from each shoot than 15 hotels) and a busy marina (4 ha) established and their length and width were measured to be divided in 1979 accommodating about 340 boats of different in the following categories: adult leaves (length greater categories. In 1993, a seawage was built at 2 km than 50 mm with sheath), intermediate leaves (length South of the sampling site. The facility includes physical, chemical and biological processes to treat
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Table I. Posidonia oceanica : features of sampling stations, sediment type, meadow type and potential stress resources (He: Hergla; EK: El Kantaoui; Mo: Monastir; Ma: Mahdia).
GPS location
Depth (m) N E Sediment type Meadow type Potential stress He 2 36° 02' 165'' 10° 30' 595'' Sand Barrier reef, presence of micro-atoll Offshore tuna farm 5 36° 02' 226'' 10° 30' 654'' Sand Undulating meadow Coastal port 10 36° 02' 596'' 10° 30' 877'' Sand Continuous meadow EK 2 35° 53' 606'' 10° 36' 052'' Sand Continuous meadow, presence of dead matte Marina 5 35° 53' 912'' 10° 36' 540'' Sand Continuous meadow Coastal constructions 10 35° 53' 920'' 10° 36' 556'' Sand Continuous meadow Water treatment plant Mo 2 35° 47' 194" 10° 49' 593" Sand Continuous meadow Marina 5 35° 47' 182" 10° 50' 080" Sand Continuous meadow Coastal constructions 10 35° 47' 199" 10° 50' 520" Sand Continuous meadow Sea port
Ma 2 35° 30' 500'' 11° 04' 979'' rock Continuous meadow Coastal constructions 5 35° 30' 640'' 11° 04' 481'' Sand Continuous meadow Sea port 10 35° 30' 485'' 11° 04' 258'' Sand Continuous meadow
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Bull. Inst. Natn. Scien. Tech. Mer de Salammbô, Vol. 38, 2011 greater than 50 mm without sheath) and juvenile leaves meadows at the three depths (Table IV). Coefficient A (length less than 50 mm without sheath) . showed difference among localities, with Monastir Two leaf indices were calculated: showing less damaged leaves (Table IV). • The leaf Area Index , which corresponds to the leaf The shoot biomass and epiphyte weight per shoot surface area for 1 m2 meadows (in m2 m-2); showed differences both among localities and depth • The coefficient A , which indicates the number of leaves (Table IV). Mahdia exhibited the highest values of shoot having lost their apex (due to grazing by herbivores or biomass at -5m and -10m and El Kantaoui the lowest due to hydrodynamic action). one and at - 2m. Hergla showed the highest shoot The epiphytic b iomass was estimated after scraping biomass and Monastir the lowest shoot biomass. epiphytes from the leaves using a razor blade. Then, The highest epiphytic biomass was recorded at El epiphytes were dried in an oven at 70°C for 48 h and Kantaoui whereas the lowest one was observed at weighed. The scraped leaves were then dried at 70°C for Mahdia. No significant differences were found in annual 48 h and weighed (± 0.1 mg) in order to estimate mean leaf production among localities or depths. shoot dry weight. Rhizome growth showed significant differences among The annual rhizome growth (cm year-1) and the leaf localities (Table IV). Mahdia and Hergla had rhizome formation rate (number of leaves year-1) were growing faster than Monastir and El Kantaoui. determined for the last decade (1994-2003) following the standardized procedure of lepidochronology analysis (Pergent and Pergent-Martini, 1991; Pergent et al., DISCUSSION 1995). Data on structure, morphology and shoot growth of P. Statistical analysis oceanica along the eastern Tunisian coast have been The two-way ANOVA was performed for the meadows lacking until now except of scarce studies (El Asmi- (four levels, random) and depths (three levels, Djellouli ., 2004; Pergent and Zaouali, 1992; Pergent orthogonal to meadows) for the variables (shoot density, and Pergent-Martini, 1990). number of adult leaves per shoot, number of Particular and rare structures of P. oceanica were intermediate leaves per shoot, number of juvenile leaves observed in the upper limit of Hergla. These more or per shoot, mean adult leaf length, mean adult leaf width, Leaf Area Index, coefficient A, shoot biomass, epiphyte less circular spot of P. oceanica in very shallow water weight, annual leaf production and rhizome growth). For were called 'micro-atolls'. These particulars structures significant difference among variables were analyzed can be linked to hydrodynamic action, currents and/or with Tukey’s multiple comparison tests to determine water temperature (Boudouresque et al., 2006). which variables were significantly different. Data were Micro-atolls have been also found in four sites in the tested for normality and homogeneity of variance by Mediterranean Sea, Turkey (Boudouresque et al., Kolmogorov-Smirnov test (Sokal and Rohlf, 1981) to 1990), Marsala (western Sicily) (Calvo and Fradà- meet the assumptions for parametric statistics. Orestano, 1984), Saint-Florent (Corsica) (Pasqualini RESULTS et al., 1995), Bibane Lagoon (Riveill et al., 2006) and Structural, morphological and shoot growth parameters Ain Al-Ghazala (Libya) (Pergent et al., 2007). of P. oceanica of the four Tunisian meadows are According to the grading scale of Pergent et al. presented in Table II. Data on shoot density and (1995), El Kantaoui meadow, which exhibited the meadow cover differed significantly among localities lowest densities, should be qualified as abnormal. and depths. El Kantaoui showed the lowest densities and cover. Monastir meadow, also with low shoot densities, is Number of adult, intermediate and juvenile leaves per considered therefore from abnormal to subnormal. shoot differed significantly between localities with less Hergla values range from subnormal to normal, than three photosynthetically active leaves per shoot at - whereas Mahdia, with the highest values, is 10 m at El Kantaoui (2.8 ± 0.4 leaves per shoot). considered as normal. P. oceanica meadows of The number of intermediate leaves was significantly Mahdia -2m, characterised by the highest density, lower at El Kantaoui meadow, as opposed to the number could be the result of a different substrate (Zakhama of juvenile ones, the most abundant. and Charfi, 2005). In fact, rocky substrate can affect Significant differences were found in adult leaves length the morphological and growing variables of seagrass and width both between localities and depth (Table III). (Marbà and Duarte, 1997; Mils and Fonseca, 2003; El Kantaoui and Monastir meadows showed the shortest Peirano et al., 2005; Di Carlo et al., 2006). The adult leaf length while Mahdia exhibited the longest one, lowest density and meadow cover were recorded at El except at -2m depth (Table III). Kantaoui was probably related to pollution situation The leaf width decreased with the depth at El Kantaoui as demonstrated by the high concentration of organic and increased at Mahdia. El Kantaoui and Monastir showed the widest leaves at -2m. carbon and micropollution detected in the sediment The LAI differed significantly between localities and layer (Pavoni et al., 2000). These conditions could depth; indeed, a significant interaction between localities affect negatively the growth and survival of seagrass and depth was observed. LAI was significantly low at El species (Terrados et al., 1999). Kantaoui, the highest value was recorded at Mahdia
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Table II. P. oceanica health parameters recorded in the four sampling localities at 2, 5 and 10 m of depth.
-2m -5m -10m He EK Mo Ma He EK Mo Ma He EK Mo Ma Cover (%) 95 ± 1,9 50 ± 8,5 75 ± 6,1 95± 3,7 80 ± 6,4 70 ± 11,8 75 ± 11,8 90 ± 5,3 60 ± 7,3 25 ± 7,5 50 ± 6 85 ± 9,3 Shoot density (shoot m -2) 631 ± 16 388 ± 42 456 ± 33 894 ± 160 496 ± 29 362 ± 14 402 ± 22 487 ± 40 267 ± 15 144 ± 20 221 ± 16 441 ± 61 Number of adult leaves per shoot 2.7 ± 0.3 2.8 ± 0.3 2.4 ± 0.5 3.4 ± 0.3 3 ± 0.4 2.7 ± 0.3 2.4 ± 0.3 3.7 ± 0.3 3 ± 0.4 2.4 ± 0.3 2.9 ± 0.2 3.7 ± 0.3 Number of intermediate leaves per shoot 2.2 ± 0.3 1 ± 0.2 3.1 ± 0.2 2.5 ± 0.5 2.1 ± 0.4 1.5 ± 0.3 2.6 ± 0.2 2.0 ± 0.2 1.8 ± 0.3 0.4 ± 0.3 2.7 ± 0.4 1.8 ± 0.3 Number of juvenile leaves per shoot 1.4 ± 0.2 2.7 ± 0.2 0.4 ± 0.2 1.0 ± 0.4 1 ± 0.2 2.2 ± 0.2 0.9 ± 0.2 0.7 ± 0.4 1.5 ± 0.4 2.5 ± 0.3 0.9 ± 0.3 0.7 ± 0.4 Mean adult leaf length (mm) 347 ± 26 242 ± 29 253 ± 17 294 ± 22 379 ± 36 159 ± 13 242 ± 12 418 ± 37 301 ± 43 200 ± 29 245 ± 22 607 ± 61 Mean adult leaf wide (mm) 9.5 ± 0.1 11 ± 0.2 10.5 ± 0.3 8 ± 0.2 9.4 ± 0.1 9.5 ± 0.1 9.5 ± 0.2 9 ± 0.1 9.7 ± 0.2 9.3 ± 0.3 10.3 ± 0.3 9.7 ± 0.3 Leaf Area Index (m² m-²) 7.8 ± 0.2 3.8 ± 0.4 4 ± 0.1 11 ± 1.4 7.2 ± 0.4 2.1 ± 0.1 2.2 ± 0.1 8.5 ± 0.5 2.6 ± 0.1 0.7 ± 0.1 1.9 ± 0.1 13 ± 1.3 Coefficient A (%) 40.7 60.8 28 70 42.3 51.5 34.8 46 48 60 26 41 Shoot biomass (g per shoot) 0.6 ± 0.03 0.4 ± 0.03 0.3 ± 0.1 0.5 ± 0.1 0.8 ± 0.04 0.25 ± 0.03 0.3 ± 0.03 0.9 ± 0.05 0.5 ± 0.03 0.25 ± 0.02 0.35 ± 0.05 1.4 ± 0.2 Epiphyte weight (mg cm-²) 2.5 ± 0.2 5.66 ± 0.8 1.7 ± 0.1 0.31 ± 0.1 1.4 ± 0.5 2.7 ± 0.3 0.3 ± 0.03 0.26 ± 0.01 1.6 ± 0.1 3.3 ± 0.5 0.8 ± 0.1 0.5 ± 0.06 Annual leaf production (leaves year-1) 7.3 ± 0.5 6.8 ± 0.4 7.7 ± 0.3 7.7 ± 0.3 7.5 ± 0.4 7.7 ± 0.3 7.1 ± 0.2 7.5 ± 0.4 7.8 ± 0.3 6.9 ± 0.5 7 ± 0.4 7.4 ± 0.8 Mean rhizome growth (mm year-1) 6.5 ± 0.5 6.1 ± 0.6 7.2 ± 0.6 6.1 ± 0.5 9.4 ± 1.4 4.2 ± 0.7 5.4 ± 0.5 7.7 ± 2.16 7.6 ± 0.3 3.8 ± 0.5 4.5 ± 0.4 8.1 ± 1.3
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Table III. Results of two-way ANOVA (fixed factors: locality (four levels, random) and depths (three levels, orthogonal to depth)) using data of cover, shoot density, number of adult leaves per shoot, number of intermediate leaves per shoot, number of juvenile leaves per shoot, mean adult leaf length and mean adult leaf width. Tukey’s HSD post hoc comparisons with respect to the depths are showed in the bottom.
Number of adult Number of Number of juvenile Cover Shoot density Adult leaf length Leaf wide leaves intermediate leaves leaves Source of d.f. P variation Locality (L) 3 < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 Depth (D) 2 < 0.0001 < 0.0001 0.708 < 0.0001 0.024 < 0.0001 < 0.0001 (L x D) 6 < 0.0001 < 0.0001 0.229 0.008 0.010 < 0.0001 < 0.0001 Ma=Ek; Tukey test (He=Ma)**>Mo* Ma**>He**>Mo* EK**>Ma*>He**> He**>(Ma=Mo= EK**>Mo**>He 2m EK=He=Mo; Mo*>Ma**=He**>EK (Depth) *>EK *=EK Mo EK) **>Ma Ma**>(He.Mo) He=Ma**>Mo*> Ma**>(He=Ek=M Ma**>He**>(Mo (EK. He. 5m He=Ma=Me=EK Mo=Ma=He**>EK EK*>(Ma=He=Mo) EK o) =EK) Mo)**>Ma Ma=He=Mo. 10 Ma**>(He=Mo)* Ma**>He**>Mo* EK*>He*>(Ma= Ma**>He**>(Mo (Ma=He=EK); Mo=EK; Mo**>Ma**=He**>EK m * >EK *>EK Mo) =EK) Mo*>EK (Ma=He)**>EK (*p<0.05, **p<0.01)
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Table IV. Results of two-way ANOVA (fixed factors: locality (four levels, random) and depths (three levels, orthogonal to locality)) using data of Leaf Area Index, coefficient A, shoot biomass, epiphyte weight, annual leaf production and rhizome growth. Tukey’s HSD post hoc comparisons with respect to the depths are showed in the bottom.
Leaf Area Index Coefficient A Shoot biomass Epiphyte weight per Annual leaf Rhizome growth shoot production Source of d.f. P variation Locality (L) 3 < 0.0001 < 0.0001 < 0.0001 < 0.0001 0.3227 < 0.0001 Depth (D) 2 < 0.0001 0.377 < 0.0001 < 0.0001 0.6594 0.1362 (L x D) 6 < 0.0001 0.001 < 0.0001 < 0.0001 0.0348 0.0002 Tukey test 2m Ma**>He**>(Mo=EK (Ma= EK)**>He*> He**>Ma*>EK**>Mo EK**>He**>Mo**>M n.s Mo=EK=Ma=He (Depth) ) Mo a 5m Ma**>He**>(Mo=EK EK=Ma=He=Mo Ma**>He**>(Mo=EK EK**>He**>(Mo=Ma) n.s (He=Ma)*>(Mo=EK) ) ) 10m Ma**>(He=Mo)**>E EK**>(Ma=He)**>M Ma**>He**>(Mo=EK EK**>He**>(Mo=Ma) n.s (He=Ma)**>(Mo=EK K o ) ) (*p<0.05, **p<0.01)
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A reduce mean number of intermediate leaves To conclude, several studies highlighted the associated with a high juvenile ones was observed at sensitivity of P. oceanica to different human El Kantaoui. This has been recorded in Italian activities (Ruiz and Romero, 2003; Balestri et al., meadows exposed to urban and industrial wastes 2004; Gonzalez-Correa et al., 2005). The different (Balestri et al., 2004); the authors argued that high health status observed point out to different level of intermediate leaves numbers could be the result of an disturbance along the coast. The meadow of El overproduction related to physiological plant Kantaoui is clearly the worst of the four studied sites responses under stressful conditions. due to high disturbances as a result of coastal The short leaves of El Kantaoui meadows could be constructions (water treatment plant, marina). Mahdia the result of pollution due to human activities (Short meadow shows the best vitality of all the studied et al., 1995). Similar results were recorded near urban meadows. However, the results obtained at Hergla emissaries (Maggi et al., 1977) or fishing structures and Monastir meadows indicated that those meadows (Delgado et al., 1999; Ruiz et al., 2001). A second could be somewhat worst than expected. hypothesis may be suggested concerning the Our results assess the health status of P. oceanica reduction of leaves size by mechanical action. In fact, along the eastern coast of Tunisia and can be very the epiphytic development may (i) make the leaf apex useful to evaluate the average meadow status over the more fragile, leading to an increased breakage and time and to monitor their feed back as results of thus to a reduction in leaf length (Harlin, 1980), and levels of environmental changes. (ii) generate an increase in grazing (Leoni et al., 2006; Alcoverro et al., 1997). This is argued by the BIBLIOGRAPHIE coefficient A found at El Kantaoui. The important leaf width at El Kantaoui and Alcoverro T., Duarte C.M. & Romero J. 1997 – The Monastir at -2m depth could be explained by the influence of herbivores on Posidonia oceanica combined effect of the nutrient enrichment and epiphytes. Aquatic Botany , 56: 93-104. turbidity (Pergent-Martini et al., 1996). Balestri E., Benedetti-Cecchi L. & Lardicci C. 2004 – Moreover, the highest value of epiphyte biomass at El Variability in patterns of growth and Kantaoui is probably due to the eutrophic morphology of Posidonia oceanica exposed to environment (Dimech et al., 2002; Cancemi et al., urban and industrial wastes: contrasts with two 2003) induced by the water treatment plant (Piazzi et reference locations. Journal of Experimental al. 2004). The extensive development of epiphytic Marine Biology and Ecology, 308: 1-21. organisms on the leaves of seagrasses represents a Ben Brahim M., Hamza A., Hannachi I., Rebai A., source of disturbance to the plants, limiting the Jarboui O., Bouain A. & Aleya L. 2010 – quantity of light available thus of photosynthesis Variability in the structure of epiphytic (Sand-Jensen, 1977; Ben Brahim et al., 2010). assemblages of Posidonia oceanica in relation Concerning lepidochronolocial approach, our results to human interferences in the Gulf of Gabes, indicate differences between localities in terms of Tunisia. Marine Environmental Research , 70 mean rhizomes growth. According to Pergent et al. (5): 411-421. (1995), the annual rhizomes growths pass from Ben Mustapha K., Hattour A., Mhetli M., El Abed A. normal to subnormal at El Kantaoui and Monastir. & El Tritar B. 1999 – Bionomie des étages However, Hergla and Mahdia are respectively infra et circalittoral du golfe de Gabès. Bulletin considered as normal to subnormal and normal. Institut National Scientifique et Technique Differences in the rhizome growth rate could be d’Océanographie de Pêche Salammbô, Tunisie , influenced by the several local factors, which ranges 26: 5-48. from wave exposure (Guidetti et al., 2000) and Blanc J. J. & Jeudy De Grissac A. 1984 – Érosions substrate nature (Marbà and Duarte, 1997; Peirano et sous-marines des herbiers à Posidonia al., 2005) to the degree of anthropogenic disturbance oceanica (Méditerranée). In: International (Cancemi et al., 2003; Balestri et al., 2004). Workshop on Posidonia oceanica Beds (C. F. As regards to the annual leaf production, Guidetti, Boudouresque, A. Jeudy de Grissac, J. Olivier (2001) identified a pulse reduction in mean number of ed). 1: 23-28. GIS Posidonie publ. France. leaves per shoot on P. oceanica decreasing nearly Boudouresque C.F., Ballesteros E., Ben Maiz N., 20% immediately after a disturbance (the Boisset F., Bouladier E., Cinelli F., Cirik S., replenishment of a beach with terrigenous materials) Cormaci M., Jeudy De Grissac A., Laborel J., compared to periods before the disturbance. But, at Lanfranco E., Lundberg B., Mayhoub H., last in our case, no difference on mean production of Meinesz A., Panayotidis P., Semroud R., leaves was observed between meadows; according to Sinnassamy J.M., Span A. & Vuignier G. 1990 Pergent-Martini et al. (1999), all the meadows present – Livre rouge “Gérard Vuignier” des normal values. végétaux, peuplements et paysages marins
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